It is usual to operate valves or other mechanisms by the use of hydraulically driven actuators. These are essentially pistons where a hydraulic fluid under pressure is applied to one side of the piston to move the piston and through a linkage to operate the valve. Depending on the resistance of the device to be operated and any pressure which is existing on the opposing side of the piston, a particular minimum pressure level is required for successful operation. Accordingly, where the minimum pressure is in the order of 2200 psi, a pressure range of operation from 3000 to 2200 psi may be used.
In operation, a control valve near the operator is opened in order to permit hydrualic fluid to enter the actuator. As this control valve is opened, the hydraulic fluid at high pressure, which has been existing in the control line upstream of the control valve, flows into the actuator until the piston has been moved as desired.
The hydraulic fluid is normally pressurized at single location, common to the various actuators, by means of a continuously or intermittently operated pump in a system maintaining a high pressure at the source. Where the actuator is located a considerable distance from the source, significant pressure drop will occur in the hydraulic conduit or pipe which is conveying the fluid from the source to the actuator. Accordingly, as the control valve opens to accept hydraulic fluid into the actuator, the flow occurring may be at such a rate as to drop the pressure level at the actuator below that required to operate the actuator. Accordingly, operation of the actuator is delayed to such a time as the pressure can buildup with the fluid being pumped through the hydraulic line.
A conventional method of avoiding this problem is to provide an accumulator near the hydraulic actuator. This accumulator will contain a supply of hydraulic fluid at the preestablished high level. As the pressure level drops to an extent, not exceeding the minimum acceptable, hydraulic fluid is discharged from the accumulator, thereby supplying the required fluid to the actuator immediately. The continued operation of the pump from the source thereafter need only recharge, or refill, the accumulator. Accordingly, much more rapid response of the device to be operated is achieved. Accumulators for this purpose may be simply a supply of hydraulic fluid in a tank under gas pressure or may be a piston operating against a spring.
In the use of subsea actuators, the actuator is not only remote from the hydraulic supply which is at the surface, but there is also a substantial elevation difference, which was ignored in the discussion above. Accordingly, with a pressure such as 3000 psi at the surface, the actual pressure at the actuator will be increased substantially beyond that by the weight or hydrostatic head of the fluid. The actual operating pressure of the accumulator is increased since the opposite side of the piston must discharge the hydraulic fluid either against the static head of a return line or against ambient seawater pressure, where water compatible hydraulic fluid is used. Seawater at a depth of 6700 feet has a static head of about 3000 psi. Accordingly, for an effective operating pressure of 3000 psi, the actual pressure at the actuator, and therefore at the accumulator is actually 6000 psi. It follows that a gas filled accumulator pressurized to 3000 psi at the surface would have the gas compressed to one half the volume at the operating depth. Accordingly, only half the hydraulic fluid would be available, while alternately the accumulator would have to be twice as large.
An accumulator of the type which uses a compressed spring would require that the spring be compressed with an input force equivalent to 6000 psi initially. This becomes an exceedingly large and cumbersome mechanical spring system.
U.S. Pat. No. 3,987,708 entitled "Depth in Sensitive Accumulator For Undersea Hydraulic Systems", teaches a system which uses a conventional gas charged accumulator with the high gas pressure providing the motive force for the accumulator. It is, however, depth compensated by means of a small hydraulic piston having one side open to the ambient, or sea pressure to provide depth compensation. This avoids the problem of the increased compression of the accumulator gas, but still requires that the accumulator be prechanged to full gas pressure at the surface. It also contains extremely high pressure gas which must be sealed over a long period of time.